A device is known for removing cataracts which comprises a source of laser radiation with a laser tip and means for irrigation-aspiration provided with a wedge-shaped tip, wherein the working parts of said tips are placed in the front section of the eye when a cataract-changed lens is being removed (see, e.g., U.S. Pat. No. 5,139,504).
The following steps are carried out in order to remove a cataract. The focus of a solid-body niobium laser Nd:YAG with a wavelength of 1.06 nm is directed transcorneally to the middle layers of the lens by adjusting the sharpness of a surgical microscope to the irradiated area. A light guide conducting the laser radiation is mounted in the drawtube. The generation of 20-30 pulses of laser radiation is effected with the power of each pulse being 0.2-0.5 mW/cm kV, length-10 ns, radiation power 5-10 J, size of the light spot-50 micrometers. Right after termination of the laser action, a step-like three-profile cut is formed and capsulotomy is carried out, the tip, phacoemulsifier, is entered into the eye cavity, aspiration and generation of ultrasound are turned on within the limits of 10-30% of the total power of the apparatus.
Preliminary laser action results in mechanical destruction of the links between fibers of the lens due to the formation of a plasma cloud be having the capability of expanding at supersonic speed, which weakens the stability of the lens substance to aspiration and weakens the resistivity of the lens to ultrasound oscillations, reduces the threshold values of the ultrasound power, and brings to a minimum the negative effect of the influence of ultrasound on the eyeball.
This device does not make it possible to carry out full-fledged fragmentation of the lens through the whole thickness thereof, since upon action of radiation on the substance of the lens opacities are produced which impair visual monitoring of the course of the operation and make it difficult to apply subsequent laser actions. The process of supplying a large number of laser pulses is made more difficult in the presence of dense cataracts, since there is no reflection from the fundus of the eye and there is no possibility of determining the structure of the lens in an optical cut. The device does not make it possible to carry out full laser cleavage of the lens, and therefore it is not possible to talk about a substantial reduction in the resistivity of the lens substance to aspiration and ultrasonic oscillations.
Laser irradiation at a wavelength of 1.06 nm has a traumatic effect on eye tissues, since in that case a photoionization regime is mainly generated, this regime consisting of the formation of a plasma cloud which upon expanding generates a powerful impact wave which deforms the surrounding tissues.
When two types of destructive action are simultaneously used on the lens, the drawbacks of ultrasound action--cavitation which is difficult to monitor and is traumatic for eye tissues, are added to the drawbacks of laser action. Wherein the energetic parameters of the laser action and ultrasound oscillations increase as the density of a cataract increases, and therefore the device may be suitable for removing soft uncomplicated cataracts.
A drawback of this device is that it is not possible to work with solid lenticular masses, the operation is lengthy and there is increased traumatism related to non-optimum action of the laser radiation.
A device is known for removing cataracts which comprises a source of pulse laser radiation with a laser tip and means for irrigation-aspiration provided with a tip, wherein the working parts of said tips are placed in the anterior section of the eye during removal of a cataract-changed lens (see, e.g., a prospectus of the Premier Laser Systems Inc. firm, 5, 1996).
A laser with a wavelength of 2.94 micrometers is used in the aforesaid device, wherein the laser light guide lies inside the aspiration channel. The latter is formed by a metal tube with an internal diameter of 0.8 mm. The irrigation channel is formed due to a gap between a silicone cap placed on the metal tube. The magnitude of the gap is not more than 0.2 mm. In order to create a vacuum, a vacuum pump is used which is connected to the device by means of silicone hoses.
When turned on, the laser radiation along the light guide falls on the surface of the lens. During one pulse a local defect is formed in the lens with a depth of 3-4 micrometers and width of 400 micrometers. The laser generates from 10 to 40 pulses per second. When the substance of the lens is partially evaporated, a finely dispersed suspension is partially formed in the liquid medium and is sucked out along the aspiration channel. A physiological solution supplied along the irrigation channel is used as the liquid. The amount of liquid being fed corresponds to the amount of liquid being sucked out.
Since the laser pulses have a limited depth of penetration into the tissue of the lens, quite a long period of time, about 10-12 minutes is required for the complete destruction of the lens. This creates serious difficulties during the step of forming a crater in the lens, since it is necessary to forcefully press the tip against the surface of the lens. This creates the threat of disruption of the suspensory ligaments.
There is the problem of evacuation of the destroyed lens mass from the cavity of the eye. Laser pulses are transformed into acoustic oscillations. The direction of the acoustic oscillations is in the direction opposite to the direction of evacuation of liquid from the cavity of the eye, since laser pulses counteract suction of the lens substance into the aspiration channel. The width of the aspiration channel is small, not more than 0.2 mm. The suspension of destroyed lens substance may adhere into larger conglomerates which block the aspiration channel. Aspiration stops and it becomes impossible to continue the operation.
During fragmentation of the lens substance, the laser tip should be positioned at the level of the edge of the aspirational aperture, otherwise radiation will not reach the lens, and it is not possible to destroy the lens substance sucked against the aspiration aperture due to the fact that the divergence of the laser beam is small. Quite often there is complete blockage of the aspiration channel with undestroyed lens substance. A disbalance between the supplied and removed liquid results in a collapse of the anterior eye chamber, unstable depth of the anterior chamber, amoeboid movement of the iris, narrowing of the pupil, and disturbance of visual examination of the surgery field.
All of this results in an increase in the length of the operation which in turn increases the consumption of irrigation liquid and the possibility of traumatism, an increase in the probability of injury to the iris, the rear capsule and cornea at the moment of manipulation with the core. An increase of the temperature in the front chamber of the eye may also occur, since at the moment of blockage of the aspiration channel, liquid is not fed into the eye, and it is in particular the liquid that performs the function of removing the thermal energy. Thermal damage is especially dangerous for the cornea, since it results in irreversible changes in the corneal endothelium and subsequently to distrophy of the cornea.